1. Field of the Invention
The present invention relates to thin-walled composite structures such as polymeric matrix composite structures, and more particularly, to methods for joining mechanical attachments of various types to such structures.
2. Description of the Related Art
Composites are increasingly being used in a number of different applications. The relative light weight and high strength characteristics of ceramic matrix composites (CMCs) and polymeric matrix composites (PMCs) make such composites particularly attractive for use in, e.g., the aerospace industry wherein stringent weight and strength requirements are common. In this regard, both PMCs and CMCs have been used, where practical, in many aerospace applications. However, no materials are perfect and the complexity of the underlying design requirements often lead to structures wherein multiple different materials are needed. For example, it is desirable in many applications to use the physical properties of such materials as aluminum, tungsten, invar and other high grade aerospace metals in conjunction with such light weight composites.
The manufacture of composite tanks, pressure vessels and other structures typically require the incorporation of flanges, feed throughs, and other mechanical attachments to the basic structure. For example, a typical light weight, high strength composite tank may require metallic flanges, structural reinforcements such as struts, feed through members and other mechanical attachments so as to enable the tank, vessel or other structure to meet subsystem requirements and environmental factors. These requirements, in turn, result in the need for openings and accoutrements to be provided in the composite tank and/or in additional loading on the composite material, and thus can have a dramatic impact on the stress concentration in and around the mechanical attachments.
It is a goal in designing structures wherein such attachments are joined to the basic composite structure to distribute stresses in all directions particularly at the interface between the metal attachment and the composite. For example, in tank structure wherein, as described above, a flange is attached to the composite tank, hoop stresses translated through a flanged opening in a three-dimensional interface, rather than a two-dimensional surface, will result in high strength characteristics and a decreased potential for debonding. Similarly, a tank strut or other structural attachment distributing mass acceleration loading of a tank is more effective in a three-dimensional, rather than two-dimensional, translation form. Moreover, debonding from “out-of-phase” forces (i.e., loads perpendicular to the layups of the composite) and interlaminar shear stresses are of concern in any tank design.
Currently, most multi-material tanks, such as those described above wherein metal attachments are joined to a basic composite tank structure, are made using epoxy bonding and mechanical attachment methods. However, these methods suffer important disadvantages. For example, an epoxy bond is a two-dimensional translation form that is subject to delamination while mechanical attachments can result in leaks and in fit problems.